The proposed study is designed to clarify cell-cell interactive mechanisms whereby underlying oxygen toxicity enhances acute lung inflammation following endotoxemia. Pretreatment with hyperoxia has been found to enhance and accelerate the influx of neutrophils (PMN) into bronchoalveolar spaces, and to intensify lung damage in a previously described rat model of endotoxin-induced acute lung injury. The overall aim of this proposal is to determine whether hyperoxia affects the functions of pulmonary endothelial cells, alveolar macrophages, and blood leukocytes in ways that potentiate endotoxin-induced acute alveolar inflammation. To achieve this goal, we will determine whether hyperoxia enhances the adherence and/or accelerates the turnover of PMNs in vivo in the normal lung or in the lung injured by endotoxemia. Additional studies will test whether hyperoxia in vivo or in vitro has direct effects on alveolar macrophages and blood leukocytes resulting in altered production of chemotactic factors, migration inhibiting factors, lysosomal enzymes or superoxide anion following activation by endotoxin. Next we will separate subpopulations of alveolar macrophage by centrifugation on continuous density gradients of Percoll to determine whether functionally abnormal subpopulations of macrophages appear when oxygen, endotoxin, or both are given, and whether these subpopulations produce factors in culture which perpetuate inflammation or promote fibrosis. Finally, we will employ cultures of rat microvascular endothelial cells to detect the effects of oxidant damage to the endothelium upon adherence to and migration across an endothelial cell monolayer by neutrophils. We will also investigate whether there is a synergistic effect between underlying oxygen toxicity and leukocyte-induced injury on the integrity of endothelial cells. We will quantify damage to the endothelial cells by detection of the expression of latent Fc receptors on the endothelial cell surface. These receptors are not found on the surface of endothelial cells but become evident following cellular injury. It is anticipated that the multifaceted approach described in this application will clarify the theoretical basis of hyperoxic enhancement of endotoxin-induced alveolitis. This topic is of clinical importance because oxygen therapy and sepsis frequently coincide during treatment of acute respiratory failure, often producing a fatal outcome.